Referring to FIG. 10, a block diagram is shown of a typical thermal printer 1002. The printer 1002 contains a thermal print head (TPH) 1008 having a linear array of heating elements 1010 (also referred to herein as “print head elements”) that print on an output medium 1012 by, for example, transferring pigment from a donor sheet to the output medium 1012 or by initiating a color-forming reaction in the output medium 1012. The output medium 1012 is typically a porous receiver receptive to the transferred pigment, or a paper coated with the color-forming chemistry. Each of the print head elements 1010, when activated, forms color on the medium 1012 passing underneath the print head element, creating a spot having a particular optical density. Regions with larger or denser spots are perceived as darker than regions with smaller or less dense spots. Digital images are rendered as two-dimensional arrays of very small and closely-spaced spots.
A thermal print head element is activated by providing it with energy. Providing energy to the print head element increases the temperature of the print head element, causing either the transfer of colorant to the output medium 1012 or the formation of color in the output medium 1012. The density of the output produced by the print head element in this manner is a function of the amount of energy provided to the print head element. The amount of energy provided to the print head element may be varied by, for example, varying the amount of power provided to the print head element within a particular time interval or by providing power to the print head element for a varying time interval.
The thermal printer 1002 also contains a platen 1014 which comes into contact with the output medium 1012 as it passes through the printer 1002. The platen temperature, which may change over time, therefore affects the temperature of the medium 1012 as the medium 1012 passes under the thermal print head 1008. The temperature of the output medium 1012 affects the print density. Because the temperature of the platen 1014 affects the temperature of the output medium 1012, the platen temperature therefore indirectly affects print density, thereby causing undesirable artifacts in the output.
What is needed, therefore, are techniques for eliminating or mitigating undesirable effects of platen temperature on print density.